The present invention relates generally to reactors that generate hydrogen from hydrocarbon fuels.
Steam reforming of methanol to produce hydrogen for end use applications occurs via the following overall reaction requiring a catalyst:CH3OH+H2O3H2+CO2 ΔH=49.4 kJ/mol  (1)
This typically occurs in combination with at least two intermediate reactions:
1) endothermic (consuming heat) methanol decompositionCH3OH2H2+CO ΔH=92.0 kJ/mol  (2)
2) reversible, exothermic (releasing heat) water gas shift (WGS) reactionCO+H2OH2+CO2 ΔH=−41.1 kJ/mol  (3)
A typical reactor has a first stage that requires heat input and operates at 250-300° C. over a catalyst that favors methanol decomposition [Eqs. (1) and (2)] in the forward direction. Thus, conversion of methanol is achieved by the following net reaction (neglecting trace byproducts),CH3OH+H2O→(3−x)H2+(1−x)CO2+xCO+xH2O0≦x≦1 ΔH>0  (4)
Because the CO concentration is too high (1-4%) in the product mixture for the PEM fuel cells to tolerate (require CO<10 ppm), a second reactor stage is typically required, which is designed to operate at lower temperature (100-150° C.) over a catalyst that favors the conversion of CO into CO2 via the water gas shift (WGS) reaction [Eq. (3)]. Also, excess water vapor is often added to further shift the equilibrium away from CO towards CO2. However, because of the reversible nature of the WGS reaction, it is impossible to convert 100% of the CO or even to reduce its concentration to <10 ppm levels. Hence, an additional third stage is often required to either 1) preferentially oxidize the CO to acceptable levels or 2) purify the hydrogen by separation through a hydrogen selective membrane.
Technology for large scale steam reforming of methanol is quite mature and most reactors are of the fixed catalyst bed-type, operated in a steady-state, continuous-flow regime. While suitable for large-scale hydrogen generation, these reactors are fundamentally flawed for portable and distributed applications because of the poor process scale-up/down, sequential uni-functional design not suitable for miniaturization and system integration, and poor reaction yields due to fundamental mismatch between the time scales of the catalytic chemistry and the transport processes. For transportation and small scale distributed power applications, other important requirements include rapid startup, rapid transient response to changing power demands, high energy efficiency, purity of hydrogen (CO<10 ppm) produced, and lightweight, compact design. Further, in addition to removal of CO from the product, the potential for pre-concentration of the resulting greenhouse gas CO2 for on-board sequestration is an appealing opportunity to reduce the environmental impact of the transportation sector. So far, it has proven difficult to meet these requirements through simple miniaturization of traditional reactor designs and processes. Thus, there is significant interest in development of small-scale, highly scalable reactors for producing hydrogen from synthetic (or natural) hydrocarbon fuels for fuel cell power plants with widely varying power generation requirements (e.g. less than 1 W to more than 100 kW). This has resulted in strong demand for revolutionary new approach(es) to reactor designs which feature scalability, multi-functionality, and hyper-integration of the required system components.
It would be desirable to have improved reactors for generating hydrogen from hydrocarbon fuels.